Evaluation of the impact of Mn and Al on the microstructure of Fe–Co–Ni–Cr based high entropy alloys

In this study, an equiatomic FeCoNiCrMn high entropy alloy (HEA) was examined and compared to two Al-containing variants, FeCoNi1.5CrMnAl0.5 and FeCoNi1.5CrAl0.5, in terms of microstructure, phase composition and hardness. The vacuum cast, hot isostatically pressed (HIP'd) alloys were evaluated using differential scanning calorimetry (DSC), SEM, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Vickers microhardness testing. The DSC results indicated that the solidus and liquidus temperature of the base alloy on cooling are 1296.3 and 1314.8°C, respectively. Its microstructure exhibited a single-phase face centred cubic (FCC) structure, with a small amount of σ-phase. The two Al-containing alloys (FeCoNi1.5CrMnAl0.5 and FeCoNi1.5CrAl0.5) were observed to form a secondary phase, in a FCC matrix, along the grain boundaries and within the grains. FeCoNi1.5CrMnAl0.5 had a solidus and liquidus temperature at 1271.8°C and 1287.6°C upon cooling, while FeCoNi1.5CrAl0.5 exhibited a solidus at 1354.7°C and liquidus at 1376.1°C. The hardness of the base composition was measured at 142 HV while the addition of Al and increase of Ni content in FeCoNi1.5CrMnAl0.5 increased the hardness to 202 HV and the removal of Mn in FeCoNi1.5CrAl0.5 further enhanced the hardness to 217 HV. These results aim to form a basis for understanding the effects of HEA microstructure on the material performance in oxidising environments, such as supercritical-CO2.